1. Field of the Invention
The present invention relates to a method for manufacturing a SIMOX (Separation by IMplanted OXygen) wafer based on a SIMOX technology among methods for manufacturing an SOI (Silicon On Insulater) wafer having a single-crystal silicon layer formed on a silicon single-crystal main body via an oxide film.
2. Description of the Related Art
As conventional methods for manufacturing an SOI wafer, there are known a bonding method for bonding two silicon wafers via an oxide film and a SIMOX method for implanting an oxygen (O+) ion from a silicon wafer surface to form an ion-implanted layer serving as an oxygen layer having a high concentration in a region at a predetermined depth in a wafer and applying a heat treatment to this wafer to change the ion-implanted layer into a buried oxide film (which will be referred to as a BOX layer hereinafter) In particular, an SOI wafer manufactured by the SIMOX method is called a SIMOX wafer.
A method for manufacturing a SIMOX wafer at an initial stage of development is based on a high-dose technology. In this method for manufacturing a low-dose SIMOX wafer, oxygen atoms are implanted into a silicon wafer with an implantation energy of approximately 200 keV at the rate of approximately 2×1018 atoms/cm2, a BOX layer is formed in the wafer in an ion-implanted state (as-implanted state), and then high-temperature annealing treatment is carried out. A crystal defect that has occurred in the SOI layer by this annealing treatment can be remedied, and the BOX layer can be reformed.
However, since in this high-dose technology for manufacturing the SIMOX wafer an oxygen ion dose amount is large, there has been a problem that many threading dislocations occur in the SOI layer, an ion implantation time is long and a manufacturing efficiency is poor, for example. This threading dislocation brings a leak current or deterioration in a heterointerface when a device is manufactured. As a result, an improvement in device performance or development of functionality may be obstructed.
Therefore, a low-dose technology has been developed in order to inhibit threading dislocations from being generated in an SOI layer and reduce an ion implantation time. In this method for manufacturing a low-dose SIMOX wafer, oxygen atoms are implanted from a surface of a silicon wafer at the rate of approximately 3.5×1017 to 4.5×1017 atoms/cm2 with an implantation energy of approximately 180 keV, and then a high-temperature heat treatment is carried out to form a BOX layer to be continuous in a surface direction parallel to the surface of the silicon surface. In a case where the implantation energy is 180 keV, the BOX layer that is continuous in the surface direction parallel to the wafer surface can be formed only when a dose amount is 3.5×1017 to 4.5×1017 atoms/cm2. A fluctuation range of this dose amount is called a dose window. In this high-dose technology for manufacturing the SIMOX wafer, a density of threading dislocations in the SOI layer can be reduced and an implantation time can be reduced to improve a manufacturing efficiency.
However, since the SIMOX wafer obtained based on this low-dose technology has a small oxygen ion dose amount, a film thickness of the BOX layer becomes thin, and reliability of the BOX layer is dubious. Further, in a case where a film thickness of the BOX layer to be formed is thin, when a particle adheres to a silicon wafer surface at the time of ion implantation, this particle functions as a mask so that an unimplantable part is apt to be generated in the ion-implanted layer formed in the silicon wafer. Although the ion-implanted layer becomes the BOX layer by annealing treatment, the unimplantable part serves as a pin hole that is one type of crystal defects to reduce electrical insulation properties. There has been a problem that a percentage of this pin hole density is higher than that of a high-dose SIMOX wafer, for example.
Thus, in order to inhibit generation of the pin hole in the BOX layer, there have been proposed a method for manufacturing an SOI substrate called an ITOX (Internal Thermal OXidation) technology where annealing treatment is performed to an ion-implanted silicon wafer and then oxidizing treatment is carried out in a high-temperature oxygen atmosphere, and an SOI substrate manufactured thereby (see, e.g., Japanese Unexamined Patent Application Publication No. 263538-1995(claim 1, claim 6, paragraphs [0009], [0010], [0025], and [0026], FIG. 1)).
In the method disclosed in Japanese Unexamined Patent Application Publication No. 263538-1995, oxygen atoms are first implanted from a surface of a silicon wafer at the rate of approximately 0.4×1018 atoms/cm2 with an implantation energy of approximately 180 keV. Then, the wafer having an ion-implanted layer formed therein is subjected to annealing treatment in an inert gas atmosphere containing oxygen whose concentration is less than 1.0% to change the ion-implanted layer into a BOX layer, and then this wafer is further subjected to a high-temperature heat treatment in an atmosphere containing high-concentration oxygen whose concentration exceeds 1%. When the wafer is subjected to the high-temperature heat treatment, high-concentration oxygen in the atmosphere enters the wafer from front and rear surfaces of the wafer to be diffused. When this oxygen stays to be laminated as SiO2 at a BOX layer interface portion, the BOX layer grows, thereby increasing a film thickness of the BOX layer. In a SIMOX wafer obtained by this ITOX technology, when SiO2 is laminated at the BOX layer interface portion, a pin hole generated in the BOX layer can be remedied, thus reducing a pin hole density. Furthermore, roughness of an interface between the BOX layer and the silicon wafer can be improved. As a result, electrical characteristics of a device can be homogenized.
However, even in this manufacturing method based on the ITOX technology, since an oxygen ion dose amount is as large as approximately 0.4×1018 atoms/cm2, an ion implantation time is long. Moreover, a high-temperature oxidizing treatment is required in addition to the annealing treatment, and hence there is a problem that the manufacturing efficiency is poor and the productivity is lowered.
Thus, in order to reduce the ion implantation time, there has been proposed a method for manufacturing an SOI wafer called an MLD (Modified Low Dose) technology where oxidizing treatment is applied to a wafer having two ion-implanted layers including an amorphous layer formed therein (see, e.g., Specification in U.S. Pat. No. 5,930,643 (claim 1, specification p. 1, the second right column, 11. 5 to 43, FIG. 1(a)).
In the method disclosed in U.S. Pat. No. 5,930,643, ion implantation is divided into two processes, and oxygen atoms are implanted into a wafer having different temperatures each time. As a result, two implanted layers in different states, i.e., a high-concentration oxygen layer and an amorphous layer are formed in the wafer, and then this wafer is subjected to a high-temperature oxidizing treatment in a mixed gas atmosphere.
Specifically, in a state where the silicon wafer is heated, oxygen ions are first implanted at the rate of 2×1017 atoms/cm2 with an implantation energy of 185 keV to form a first ion-implanted layer serving as the high-concentration oxygen layer in the wafer. Then, in a state where this wafer is cooled, the oxygen ions are implanted at the rate of 3×1014 atoms/cm2 with an implantation energy of 185 keV to form a second ion-implanted layer in an amorphous state in such a manner that this layer becomes continuous with a surface of the first ion-implanted layer on a wafer front surface side. Additionally, when a lower portion of the second ion-implanted layer overlaps an upper portion of the first ion-implanted layer, high-concentration oxygen is contained in the amorphous layer. Further, a temperature of this wafer is increased in an inert atmosphere, e.g., argon containing 0.1 to 10% oxygen, and then this wafer is maintained at a high temperature in an atmosphere containing oxygen of 5 to 100% oxygen containing argon or the like. Maintaining the wafer at a high temperature in this atmosphere containing oxygen changes the first ion-implanted layer into a BOX layer. It is to be noted that the second ion-implanted layer contains high-concentration oxygen in the amorphous layer thereof, and hence re-crystallization does not smoothly advance at the time of increasing a temperature, and the second ion-implanted layer becomes a high-density defective layer including polycrystal, a twin crystal, or a stacking fault. A region in which this defective layer is formed has a property that oxygen readily precipitates. In a subsequent process of maintaining the wafer at a high temperature in the atmosphere containing oxygen, oxygen in the atmosphere containing oxygen enters the wafer from front and rear surfaces of the wafer to be diffused in the wafer, and oxygen is concentrated in this high-density defective layer to separate out as SiO2. The high-density defective layer having SiO2 precipitated therein becomes the BOX layer in this manner. Therefore, an ion dose amount that is as small as 2×1017 atoms/cm2 allows acquisition of a SIMOX wafer having a BOX layer whose thickness is the same as that obtained when implanting a double dose amount.
On the other hand, a SIMOX wafer having a BOX layer with a thin film thickness may be required depending on a semiconductor device manufacturer. However, according to the manufacturing method disclosed in U.S. Pat. No. 5,930,643, since oxygen in a high-temperature heat treatment atmosphere is apt to be precipitated in a high-density defective layer, production of a SIMOX wafer having a BOX layer with a thin film thickness is difficult.